Optical imagers that are used for sortation and inspection illuminate the objects to be inspected (work pieces) with one or more emitters or radiant energy sources. The work pieces reflect or transmit the light to one or more receivers, which detect the combined results of the reflected and transformed emissions. From the received signal, certain features are imaged and based on the image, a decision is made for each and every work piece about how it should be processed or sorted. The image is displayed on a visible monitor and the decisions are communicated to an actuator for realization. For example, in bulk material benefication, high grade work pieces are optically sensed and sorted out from those of lower grade. The information gathered by the sensor is typically used as a control input for an industrial process.
The descriptive features extracted from the work pieces are selected according to the nature of the work pieces, the differences between work pieces and non-work pieces, and the needs of the process. For example, when inspecting metal parts for the presence of contaminative rust, the sorter optics are set to be sensitive to the characteristic spectral signature of hematite. For sorting glass cutlet during recycling, visible color is the discriminatory feature and the optics are tuned to the visible spectrum. Optical tuning is well known and is commonly done through a judicious selection of emitters, optical filters, and receivers. A popular light source is the light emitting diode (LED), because of its stable and relatively narrow spectrum and its capacity for modulation.
In the prior art, there are physical and technological limitations that limit capacity for adaptation and usefulness. Emitters are available with only a limited number of spectral configurations, and their spectrum often does not adequately meet the requirements of the application. Also, the temporal response of many emitters is sluggish, restricting systems to continuous or unacceptably slow operation. For example, no technology exists for arbitrarily modulating an ultraviolet light source. Receiver insensitivity is also a common problem, especially in noisy environments and when high speed operation is required. Another limitation of the prior art is that the high level of radiated light creates problematical side effects in the operating environment. For example, photosensitive work pieces such as predeveloped photographic film and electronic photo-etched boards are permanently damaged when struck by light. The high levels of emitted radiation also require high operating currents to be supplied, reducing the life of the emitters and limiting portability.